Foods spoil because of the growth and destructive action of minute living organisms. Because they are so small that they can be seen only with the aid of a microscope these small living bodies have been termed micro-organisms.

These micro-organisms comprise three distinct classes:

1. Molds

Every housekeeper is familiar with the gray cottony growth which rapidly forms on bread when it is stored in a moist place. This is known as Gray Mold.

Molds are of three distinct types; black mold, blue mold and gray mold. They are composed of millions of tiny cells or "spores" which are very light and easily detached. They are present in the air at all times.

Molds grow and multiply to advantage in warm, moist places. They cannot withstand high temperatures and are easily killed by heat under 212 degrees F.

2. Yeasts

All of us are familiar with another group of micro-organisms known as yeasts. These are tiny plants which act upon sugar and change it into alcohol and carbonic acid gas. Yeasts are used for this purpose in bread-making, vinegar manufacture and in the production of various fermented beverages.

They are not like molds, forming a cottony, thread-like growth, but develop cells of various shapes and sizes. These form a white sediment or cloudy growth which can be found in fermented juices. These cells are light and are thus easily carried around by currents of air. They attack products such as fruits and jellies, which contain less than 65 per cent sugar. Yeasts are easily killed by heat, a temperature of 140 degrees F. being sufficient.

3. Bacteria

These comprise the third group of "germs" or microorganisms. Bacteria prefer nitrogenous substances of low acid content found in products such as peas, beans, corn, milk and meat. Therefore they do not thrive on fruits, or vegetables which are high in acid content.

Bacteria, unlike yeasts and molds, form spores which survive temperatures above 212 degrees F., the boiling point of water. Therefore they are able to produce in canned meats and vegetables extremely poisonous compounds unless extreme car,e is taken as to cleanliness, quick handling and proper length of time for sterilization.